1854 Inorg. Chem. 2010, 49, 1854–1864 DOI: 10.1021/ic902220a
Binding of Nitric Oxide to a Synthetic Model of Iron-Containing Nitrile Hydratase (Fe-NHase) and Its Photorelease: Relevance to Photoregulation of Fe-NHase by NO Michael J. Rose, Nolan M. Betterley, Allen G. Oliver, and Pradip K. Mascharak* Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95064 Received November 10, 2009
The activity of the non-heme iron enzyme nitrile hydratase (Fe-NHase) is modulated by nitric oxide (NO). The inactive (dark form) NO-bound enzyme is activated when exposed to light via the release of NO from the iron center. In order to determine whether oxygenation of active site Fe-bound Cys-S centers are involved in this process of NO regulation, a model complex (Et4N)[(Cl2PhPepS)Fe(NO)(DMAP)] (8) has been synthesized and structurally characterized. Complex 8 does not exhibit any NO photolability. However, following oxygenation of the Fe-bound thiolato-S centers to sulfinates (with the aid of oxaziridine), the resulting complex (Et4N)[(Cl2PhPep{SO2}2)Fe(NO)(DMAP)] (9) releases NO readily upon illumination with visible light. Spectroscopic properties of 8 and 9 confirm that these species do mimic the active site of Fe-NHase closely, and the results indicate that NO photolability is related to S-oxygenation. Results of density functional theory and time-dependent DFT studies on both 8 and 9 indicate that S-oxygenation weakens Fe-S bonding and that strong transitions near 470 nm transfer an electron from a carboxamido-N/sulfinato-SO2 MO to a dπ(Fe)-π*(NO)/dz2(Fe)-σ*(NO) antibonding orbital in 9. In case of 8, strong S-Fe-NO bonding interactions prevent the release of NO upon illumination. Together, the results of this work strongly suggest that oxygenated Cys-S centers play an important role in the process of NO regulation of Fe-NHases.
Introduction The enzyme nitrile hydratase (NHase) plays an important role in the microbial assimilation of organic nitriles1 and catalyzes the conversion of nitriles into corresponding amides.2,3 The active site of the enzyme contains either a nonheme Fe(III) or a noncorrinoid Co(III) center bound to two carboxamido-N and three cysteine-S donors.4,5 Two of the three cysteine-S ligands are post-translationally modified to a sulfenato (-SO) and a sulfinato (-SO2) moiety.6 In addition to the unusual donors around the metal center, the iron-conaining NHases (Fe-NHase) can contain a *Corresponding author. Telephone: (831) 459-4251. Fax: (831) 459-2935. E-mail:
[email protected]. (1) Endo, I.; Nojiri, M.; Tsujimura, M.; Nakasako, M.; Nagashima, S.; Yohda, M.; Odaka, M. J. Inorg. Biochem. 2001, 83, 247–253. (2) (a) Kobayashi, M.; Shimizu, S. Curr. Opin. Chem. Biol. 2000, 4, 95– 102. (b) Endo, I.; Odaka, M.; Yohda, M. Trends Biotechnol. 1999, 17, 244–248. (3) (a) Harrop, T. C.; Mascharak, P. K. Acc. Chem. Res. 2004, 37, 253– 260. (b) Kovacs, J. A. Chem. Rev. 2004, 104, 825–848. (c) Mascharak, P. K. Coord. Chem. Rev. 2002, 225, 201–214. (4) Huang, W. J.; Jia, J.; Cummings, J.; Nelson, M.; Schneider, G.; Lindqvist, Y. Structure 1997, 5, 691–699. (5) Miyanga, A.; Fushinobu, S.; Ito, K.; Wakagi, T. Biochem. Biophys. Res. Commun. 2001, 288, 1169–1174. (6) Tsujimura, M.; Dohmae, N.; Odaka, M.; Chijimatsu, M.; Takio, K.; Yohda, M.; Hoshino, M.; Nagashima, S.; Endo, I. J. Biol. Chem. 1997, 272, 29454–29459.
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Fe-bound nitric oxide (NO) molecule in the coordination sphere of the metal.7,8 This form, often referred to as the “dark form”, has been purified (Fe-NHase from Rhodococcus species R312 or N771) and characterized structurally by Endo and co-workers.8 The dark form of Fe-NHase is catalytically inactive. Exposure to light causes rapid loss of NO, and the subsequent binding of water activates the enzyme for catalysis.9 Recently, the X-ray structure of Fe-NHase by Qian and co-workers revealed the presence of two sulfinato (-SO2) groups around the iron center.10 (7) (a) Endo, I.; Nojiri, M.; Tsujimura, M.; Nakasako, M.; Nagashima, S.; Yohda, M.; Odaka, M. J. Inorg. Biochem. 2001, 83, 247–253. (b) Nagamune, T.; Kurata, H.; Hirata, M.; Honda, J.; Hirata, A.; Endo, I. Photochem. Photobiol. 1990, 51, 87–90. (c) Nagamune, T.; Kurata, H.; Hirata, M.; Honda, J.; Koike, H.; Ikeuchi, M.; Inoue, Y.; Hirata, A.; Endo, I. Biochem. Biophys. Res. Commun. 1990, 168, 437–442. (8) Nagashima, S.; Nakasako, M.; Dohmae, N.; Tsujimura, M.; Takio, K.; Odaka, M.; Yohda, M.; Kamiya, N.; Endo, I. Nat. Struct. Biol. 1998, 5, 347–351. (9) (a) Odaka, M.; Fujiii, K.; Hoshino, M.; Noguchi, T.; Tsujimura, M.; Nagashima, S.; Yohda, M.; Nagamune, T.; Inoue, Y.; Endo, I. J. Am. Chem. Soc. 1997, 119, 3785–3791. (b) Noguchi, T.; Hoshino, M.; Tsujimura, M.; Odaka, M.; Inoue, Y.; Endo, I. Biochemistry 1996, 35, 16777–16781. (c) Noguchi, T.; Honda, J.; Nagamune, T.; Sasabe, H.; Inoue, Y.; Endo, I. FEBS Lett. 1995, 358, 9–12. (10) Song, L.; Wang, M. Z.; Shi, J. J.; Xue, Z. Q.; Wang, M. X.; Qian, S. J. Biochem. Biophys. Res. Commun. 2007, 362, 319–324.
r 2010 American Chemical Society
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Inorganic Chemistry, Vol. 49, No. 4, 2010
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The exact role of the S-oxygenated donors in NO photoregulation of Fe-NHase, however, remains elusive.
sented to provide insight into the origin of this photolability.
Recent modeling approaches have provided insight into the structure-function relationship of the donor atoms in NHases.3 At least three different groups have synthesized models of the NO-bound active site.11-13 Despite such success, none of these models of the dark form of Fe-NHase exhibits NO photolability. Close examination of these models of NO-bound Fe-NHase reveals that in no case have all of the donor centers been assembled around the Fe center. For example, in all cases both thiolato-S donors and NO are included in the coordination sphere of iron, but no sulfinato (-SO2) group is present.11-13 On the other hand, model complexes reported previously by us14-16 as well as Artaud and co-workers17,18 contain coordinated sulfinato groups (in different binding modes), but the absence of bound NO precluded studies on NO photolability. In the present work, we have used a model of the Fe-NHase active site derived from a designed N2S2 ligand namely, 4,5-dichloro-N,N0 -phenylenebis(o-mercaptobenzamide) (Cl2PhPepSH4; where H = dissociable amide and thiol Hs), to determine the role of S-oxygenation in the process of NO photorelease from the metal site. By using a new S-oxygenation method, we have examined the effect of thiolato-S oxygenation (to -SO2) on NO binding to the Fe(III) species (Et4N)[(Cl2PhPepS)Fe(DMAP)] (7) and on photorelease of NO from (Et4N)[(Cl2PhPepS)Fe(NO)(DMAP)] (8) and (Et4N)[(Cl2PhPep{SO2}2)Fe(NO)(DMAP)] (9). We report herein that, although 8 (a close structural mimic of the active site of NObound Fe-NHase) does not exhibit any NO photolability, S-oxygenation serves as a “chemical switch” to allow NO photorelease from 9. The results of density functional theory (DFT) calculations are also pre-
Experimental Section
(11) Schweitzer, D.; Ellison, J. J.; Shoner, S. C.; Lovell, S.; Kovacs, J. A. J. Am. Chem. Soc. 1998, 120, 10996–10997. (12) Chatel, S.; Chauvin, A. S.; Tuchagues, J.-P.; Leduc, P.; Bill, E.; Chottard, J.-C.; Mansuy, D.; Artaud, I. Inorg. Chim. Acta 2002, 336, 19–28. (13) (a) Grapperhaus, C. A.; Li, M.; Patra, A. K.; Poturovic, S.; Kozlowski, P. M.; Zgierski, M. Z.; Mashuta, M. S. Inorg. Chem. 2003, 42, 4382–4388. (b) Grapperhaus, C. A.; Patra, A. K.; Mashuta, M. S. Inorg. Chem. 2002, 41, 1039–1041. (14) Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2001, 123, 3247–3259. (15) Tyler, L. A.; Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 1999, 38, 616–617. (16) (a) Tyler, L. A.; Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2003, 42, 5751–5761. (b) Tyler, L. A.; Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2000, 39, 357–362. (17) Galardon, E.; Giorgi, M.; Artuad, I. Chem. Commun. 2004, 286–287. (18) (a) Bourles, E.; de Sousa, R. A.; Galardon, E.; Giorgi, M.; Artaud, I. Angew. Chem., Intl. Ed. Engl. 2005, 44, 6162–6165. (b) Rat, M.; de Sousa, R. A.; Vaissermann, J.; Leduc, P.; Mansuy, D.; Artaud, I. J. Inorg. Biochem. 2001, 84, 207–213.
General Procedures. All reagents including N,N-dimethylaminopyridine (DMAP), 4,5-dichlorodiaminobenzene, and thiosalicylic acid were purchased from Aldrich Chemical Co. and used without further purification. All solvents were obtained from Fisher Scientific and distilled from standard drying agents as follows: diethyl ether (Et2O) and tetrahydrofuran (THF) from Na, acetonitrile (MeCN) and dichloromethane (CH2Cl2) from calcium hydride (CaH2), dimethylformamide (DMF) from barium oxide (BaO), and methanol (MeOH) and ethanol (EtOH) from magnesium iodide (MgI2). Standard Schlenk technique was used, and solutions were degassed by the freezepump-thaw method. NO gas was supplied by Spectra Gases Inc. and purified as previously described.19 The starting material 2,20 -dithiosalicyl chloride and the ligand PhPepSH4 were synthesized as previously described,20 while the ligand Cl2PhPepSH4 was synthesized according to a slightly modified procedure (see Ligand Synthesis). Ligand Synthesis. Cl2PhPepS2. A batch of 2,20 -dithiosalicyl chloride (2.00 g, 5.83 mmol) was dissolved in 20 mL of CH2Cl2 and filtered through a cintered glass frit to remove trace amounts of the acid. Separately, a mixture of 4,5-dichloroo-phenylenediamine (1.03 g, 5.83 mmol) and excess triethylamine (Et3N, 1.76 g, 17.5 mmol) was prepared in 25 mL of CH2Cl2. The diamine/Et3N mixture was quickly added to a stirred solution of the acid chloride. The resulting solution was stirred for 12 h, after which the solvent was removed in vacuo. The brown residue was treated with 5 mL of MeOH, and the resulting slurry stirred for 20 min. This beige solid was collected by filtration and washed with 20 mL of both cold MeOH and dry Et2O. The yield of the disulfide macrocycle Cl2PhPepS2 was 60% (1.55 g). Selected IR bands (potassium bromide, KBr matrix, cm-1): 3 430 (s, νNH), 1 665 (vs, νCO), 1 586 (m), 1 490 (vs), 1 374 (m), 1 134 (s), 1 037 (s), 904 (s), 743 (m), 476 (s). Cl2PhPepSH4. A batch of Cl2PhPepS2 (1.55 g, 3.46 mmol) was mixed with 70 mL of distilled THF, and the resulting beige slurry was degassed by the freeze-pump-thaw technique. Upon warming to 0 °C, small portions of sodium borohydride (NaBH4, 0.66 g, 17.3 mmol) were anaerobically added over several minutes, and the solution was periodically flushed with dry N2 gas. The resulting dark-green slurry was then stirred overnight at room temperature. The next day the volume of the reaction mixture was reduced to 10 mL and diluted later with 10 mL of degassed H2O. It was then neutralized with ∼6 mL of 6 M acetic acid when the protonated ligand precipitated out as a beige solid. The ligand was collected by filtration and washed successively with 10 mL portions of H2O, cold MeOH, and Et2O. Yield: 1.09 g (70%). Selected IR bands (KBr matrix, cm-1): 3 372 (w, νNH), 2 568 (w, νSH), 1 644 (vs, νCO), 1 588 (vs), 1 530 (vs), (19) Patra, A. K.; Rose, M. J.; Murphy, K. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2004, 43, 4487–4495. (20) (a) Harrop, T. C.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2006, 45, 3424–3436. (b) Harrop, T. C.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2004, 126, 14714–14715.
1856 Inorganic Chemistry, Vol. 49, No. 4, 2010 1 490 (vs), 1 452 (m), 1 369 (vs), 1 322 (vs), 1 262 (m), 1 137 (w), 879 (m), 742 (vs), 472 (w). 1H NMR at 298 K in d 6-dimethyl sulfoxide (DMSO), (δ from tetramethlysilane, TMS): 10.19 (s 2H), 8.03 (s 2H), 7.76 (d 2H), 7.49 (d 2H), 7.36 (t 2H), 7.23 (t 2H), 5.35 (s 2H). Syntheses of the Complexes. (Et4N)2[(Cl2PhPepS)Fe(Cl)] (1). A batch of Cl2PhPepSH4 (0.243 g, 0.54 mmol) was dissolved in 15 mL of degassed DMF at 0 °C and deprotonated with 0.058 g of NaH (2.43 mmol, added in small portions). Next, a solution of (Et4N)[FeCl4] (0.177 g, 0.53 mmol) in 5 mL of degassed DMF was added to it, and the resulting dark-red-brown solution was stirred overnight at room temperature. The next day, the solvent was removed in vacuo, and a solution of Et4NCl (0.179 g, 1.08 mmol) in 30 mL of MeCN was added to it. After 30 min of stirring, the mixture was filtered to remove sodium chloride (NaCl), and the filtrate was concentrated to 5 mL. A batch of 15 mL of degassed Et2O was added, and the mixture was stored at -20 °C. The brown microcrystalline product, obtained after 72 h, was collected by filtration and washed several times with small portions of Et2O. Yield: 220 mg (56%). Anal. calcd for C36H50Cl3FeN4O2S2: C 54.23, H 6.33, N 7.03; found: C 54.38, H 6.23, N 6.95. Selected IR bands (KBr matrix, cm-1): 2 980 (w), 1 585 (s, νCO), 1 569 (s), 1 533 (m), 1 478 (vs), 1 453 (vs), 1 391 (w), 1 311 (vs), 1 278 (m), 1 172 (w), 1 035 (w), 999, (w), 940 (w), 791 (w), 744 (m). UV/vis in MeCN, λ in nm (ε in M-1 cm-1): 325 (19 070), 490 (2 800), 600 sh (2 280). Value of μeff (298 K, polycrystal): 3.81 μB. (Et4N)2[ μ-S2-{(Cl2PhPepS)Fe(NO)}2] (2). A degassed solution of 1 (0.070 g, 0.088 mmol) in 5 mL of MeCN was stirred vigorously at room temperature, and NO(g) was bubbled through it. A dark microcrystalline material appeared within 5 min. The reaction mixture was then stored at 4 °C for 30 min. The product was finally collected by filtration, washed several times with Et2O, and dried in vacuo. Yield: 32 mg (60%). Anal. calcd for C56H60Cl4Fe2N8O6S4: C 50.84, H 4.57, N 8.46; found: C 50.58, H 4.86, N 8.69. Selected IR bands (KBr matrix, cm-1): 3 434 (m), 2 985 (w), 1 837 (vs, νNO), 1 585 (vs, νCO), 1 537 (vs), 1 453 (vs), 1 336 (vs), 1 182 (w), 1 111 (w), 1 084 (w), 950 (w), 915 (w), 787 (w), 750 (m), 677 (w), 574 (w). UV/vis in DMF, λ in nm (ε in M-1 cm-1): 560 (5 000), 990 (3 030). (Et4N)2[(Cl2PhPepS)Fe(NO)] (3). A solution of 5 mg (0.025 mmol) of (Et4N)[p-ClC6H4S] in 2 mL of MeCN was slowly added to a solution of 1 (20 mg, 0.025 mmol) in 2 mL of MeCN. Next, 4 mL of degassed Et2O was added, and the reaction mixture was cooled to -40 °C. One equiv of NO(g) (0.8 mL, 0.025 mmol) was then introduced into the head space via a gastight syringe, and the Schlenk flask was stored at -40 °C for 48 h. The dark-green microcrystals that separated were collected by filtration and dried in vacuo. Yield: 15 mg (80%). Anal. calcd for C36H50Cl2FeN5O3S2: C 54.61, H 6.37, N 8.85; found: C 54.85, H 6.54, N 8.79. Selected IR bands (KBr matrix, cm-1): 1 631 (s νNO), 1 584 (vs νCO), 1 512 (s), 1 452 (vs), 1 332 (vs), 792 (w), 745 (w). (Et4N)[(Cl2PhPepS)Fe(py)] (4). A solution of 8 mg (∼2 equiv) of pyridine in 5 mL of MeCN was added to a solution of 0.100 g (0.120 mmol) of 1 in 5 mL of MeCN at room temperature. The color of the original brown-red solution rapidly changed to green. Next, a solution of 23 mg (0.120 mmol) of AgBF4 in 5 mL of MeCN was added, and the mixture was stirred at room temperature for 10 min. It was then filtered (through a Celite pad) to remove the silver chloride (AgCl) precipitate, and the volume of the filtrate was reduced to ∼3 mL. Following a thorough cooling of this solution at -40 °C, a batch of 15 mL of degassed Et2O was added slowly when the dichroic red-green microcrystalline solid separated out. The product was collected by filtration and dried in vacuo. Yield: 55 mg (60%). Anal. calcd for C33H35Cl2FeN4O2S2: C 55.78, H 4.97, N 7.88; found: C 55.59, H 5.24, N 7.85. IR bands (KBr matrix, cm-1): 1 591 (s, νCO), 1 455 (s), 1 310 (m), 1 261 (s), 941 (w), 800 (s), 746 (w), 696 (w). UV/vis in MeCN, λ in nm (ε in M-1 cm-1): 320 (8 230), 610 (1 650).
Rose et al. (Et4N)[(Cl2PhPepS)Fe(1-Me-Im)] (5) and (Et4N)[(Cl2PhPepS)Fe(4-NH2-py)] (6) were synthesized by following a similar procedure. (Et4N)[(Cl2PhPepS)Fe(DMAP)] (7). A solution of 18 mg (0.15 mmol) of N,N-dimethyl-4-aminopyridine (DMAP) in 1 mL of MeCN was slowly added to a solution of 40 mg (0.050 mmol) of 1 in 5 mL of MeCN. The dark-green solution, thus obtained, did not require the addition of silver tetrafluoroborate (AgBF4) to isolate the DMAP adduct. An addition of 10 mL of degassed Et2O and a slow cooling at -40 °C afforded dark-green microcrystals of the desired product in 72% yield. Anal. calcd for C35H40Cl2FeN5O2S2: C 55.78, H 5.35, N 9.29; found: C 55.53, H 5.68, N 9.12. IR bands (KBr matrix, cm-1): 1 614 (vs, νCN/DMAP), 1 590 (vs, νCO), 1 535 (s), 1 456 (vs), 1 309 (vs), 1 225 (m), 1 009 (m), 943 (w), 745 (m). UV/vis in MeCN, λ in nm (ε in M-1 cm-1): 475 sh (2 390), 590 (1 930). Value of μeff (298 K, polycrystal): 3.74 μB. EPR spectrum, frozen MeCN/toluene glass (125 K): g = 4.39, 1.98. (Et4N)[(Cl2PhPepS)Fe(NO)(DMAP)] (8). Method A. A small batch of 7 (0.020 g, 0.025 mmol) was dissolved in 4 mL of degassed MeCN:Et2O (1:3) mixture. The green solution was then cooled to -40 °C, treated with 1 equiv of NO gas in the headspace of the Schlenk flask, and stored at -40 °C for 72 h. The red crystals (suitable for diffraction studies), thus obtained, were collected by filtration, washed several times with Et2O, and dried under vacuum. Yield: 12 mg (61%). Anal. calcd for C35H40Cl2FeN6O3S2: C 53.64, H 5.15, N 10.72; found: C 53.38, H 5.34, N 10.81. Selected IR bands (KBr matrix, cm-1): 1 850 (vs, νNO), 1 625 (s, νCN/DMAP), 1 583 (s, νCO), 1 529 (vs), 1 454 (vs), 1 336 (vs), 1 230 (m), 1 182 (w), 1 062 (w), 1 012 (m), 806 (w), 793 (w), 750 (m). UV/vis in MeCN, λ in nm (ε in M-1 cm-1): 565 (2 570), 970 (1 710). Method B. Complex 8 can be conveniently synthesized in a larger scale at room temperature by this method. A batch of 1 (200 mg, 0.262 mmol) was dissolved in 3 mL of MeCN and to it was added 150 mg (0.750 mmol) of DMAP (3 equiv). Next, 6 mL of degassed Et2O was slowly added to the reaction mixture, and the green solution was vigorously stirred at room temperature. As the solution was stirred, a stream of dry NO(g) was allowed to bubble through it. Within several min, 8 separated out of the reaction mixture as a violet microcrystalline solid. Yield: 150 mg (65%). Since 8 cannot be recrystallized from common organic solvents (it undergoes NO dissociation), method A was required for obtaining crystals of 8 suitable for the structural studies. However, for bulk measurements (IR, reactivity studies, and in the synthesis of 9), samples obtained via method B were used. (Et4N)[(Cl2PhPep{SO2}2)Fe(NO)(DMAP)] (9). A batch of 8 (180 mg, 0.23 mmol) was dissolved in 120 mL of CHCl3 at -40 °C containing 84 mg (0.69 mmol) of DMAP. Next, a solution of 211 mg (0.92 mmol) of (1S)-(þ)-(10-camphorsulfonyl)-oxaziridine in 6 mL of CHCl3 was added dropwise to the red solution of 8 and DMAP, and the whole reaction mixture was slowly warmed up to room temperature. It was then stirred in the dark for 3 h. The resulting orange solution was filtered, and the solvent was removed under reduced pressure. Next, the residue was stirred in 2 mL of THF for 48 h at -20 °C in the dark to remove the oxaziridine reaction byproduct. The orange product, thus obtained, was collected and dried in vacuo. Yield: 170 mg (81%). The product was finally recrystallized from MeCN/Et2O mixture at -20 °C (in the dark) to obtain red flakes. Note: 9 slowly decomposes at room temperature even under N2 atmosphere and is quite sensitive to light. Anal. calcd for C35H40Cl2FeN6O7S2: C 49.59, H 4.77, N 9.92; found: C 50.38, H 5.02, N 9.81. Selected IR bands (KBr matrix, cm-1): 1 842 (vs νNO), 1 623 (s νCN/DMAP), 1 598 (s νCO), 1 457 (vs), 1 228 (s), 1 077 (m νSO), 1 042 (m νSO), 1 009 (m νSO), 791 (w), 588 (w). UV/vis in MeCN, λ in nm (ε in M-1 cm-1): 450 (3 200). ESI-MS: m/z = 721.1. The S-oxygenated ligand was identified by mass spectral measurements on solutions of 9 in MeCN containing 0.1% trifluoroacetic acid (TFA) (m/z = 519.93).
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Table 1. Summary of Crystal Data and Structure Solution Parameters for 1 3 0.5Tol, 2 3 2MeCN, 4 3 0.5MeCN 3 Et2O, 7 3 THF 3 Et2O 3 0.5MeCN, and 8 1
2
4
7
8
empirical formula C39.5H54N4O2S2Cl3Fe C60H66Cl4Fe2N10O6S4 C37H44Cl2FeN4.50O2.75S2 C42H56.5N5.5O3.5S2Cl2Fe C35H40N6O3S2Cl2Fe formula weight 843.19 1404.97 786.64 885.30 783.60 crystal color dark-red violet dichroic red/green dichroic red/green red shard size, mm 0.19 0.14 0.13 0.13 0.05 0.03 0.21 0.19 0.03 0.28 0.24 0.14 T (K) 153(2) 193(2) 153(2) 153(2) 150(2) wavelength, A˚ 0.71073 0.7749 0.71073 0.71073 0.77490 crystal system monoclinic triclinic triclinic triclinic triclinic P1 P1 P1 P1 space group P21/c a, A˚ 13.7465(17) 10.619(2) 13.4773(12) 13.4564(14) 9.6049 b, A˚ 16.541(2) 12.631(3) 16.3851(15) 18.400(2) 9.6051 c, A˚ 19.160(2) 13.651(3) 18.9445(17) 18.930(2) 19.251 R, deg 90 66.951(4) 78.9800(10) 86.3500(10) 82.72 β, deg 109.246(2) 69.002(4) 79.4000(10) 79.8400(10) 82.65 γ, deg 90 71.607(4) 72.8100(10) 79.0150(10) 88.48 4113.3(9) 1539.8(5) 3886.5(6) 4527.0(8) 1747.1(6) V, A˚3 Z 4 1 4 4 2 1.362 1.515 1.344 1.299 1.490 dcalc, g/cm3 0.702 1.058 0.673 0.588 0.944 μ, mm-1 1.031 1.044 0.993 1.035 1.060 GOFa on F2 final R indices R1 = 0.0345 R1 = 0.0496 R1 = 0.0464 R1 = 0.0613 R1 = 0.0772 [I > 2σ(I )] wR2 = 0.0772 wR2 = 0.1290 wR2 = 0.0936 wR2 = 0.1693 wR2 = 0.2046 R1 = 0.0505 R1 = 0.0731 R1 = 0.0920 R1 = 0.0958 R1 = 0.0973 R indicesb wR2 = 0.0847 wR2 = 0.1406 wR2 = 0.1110 wR2 = 0.1886 wR2 = 0.2228 all datac P P P 2 2 2 a 1/2 b (M = number of reflections, and N = number of parameters refined). R1 = | |Fo| - |Fc| |/ |Fo|. cwR2 = o - Fc ) ]/M - N)] P GOF = [ [w(FP [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2.
Diffusion of CH2Cl2 into a solution of 9 and Na-triflate in MeCN at -20 °C in the dark afforded Na-9 as a dark-red polycrystalline solid after 6-7 days. The solid was characterized by IR spectroscopy and mass spectrometry. X-ray Data Collection and Structure Solution and Refinement. Dark-red needles of 1 3 0.5 toluene were obtained via vapor diffusion of toluene into MeCN solution of the complex under inert atmosphere at ambient temperature. Violet rods of 2 3 2MeCN suitable for beamline diffraction were obtained by the slow reaction of 1 in MeCN with 1 equiv of NO in the headspace (along with N2) at 4 °C over several days. Dichroic red/green needles of 4 3 0.5MeCN/Et2O were obtained by storing a solution of 4 in MeCN:Et2O (1:5) at -20 °C for several weeks. Green blocks of 7 3 THF/Et2O/0.5MeCN were obtained by cooling a MeCN:THF:Et2O (1:5:10) solution of 7 at -20 °C for several weeks. Red shards of 8 were obtained by the reaction of 1 in MeCN with 1 equiv of NO gas in the headspace in the presence of 1 equiv of DMAP at -40 °C for 5 days. Diffraction data for 1, 4, and 7 were collected on a Bruker Apex-II instrument using MoKR radiation (λ = 0.71073 A˚), and the data were corrected for absorption. Diffraction patterns for 2 and 8 were collected at beamline 11.3.1 at the Advanced Light Source at the Lawrence Berkeley National Laboratory. All structures were solved using the standard SHELXS-97 package.21 Additional refinement details are contained in the CIF files (see Table 1 and Supporting Information). CIF files for 7 and 8 have been included in our previous communication.22 Instrument parameters, crystal data, and data collection parameters for all the complexes are summarized in Table 1. Selected bond distances and bond angles for 1, 2, 4, 7, and 8 are listed in Table 2. Physical Measurements. A Perkin-Elmer Spectrum-One FTIR spectrometer was used to obtain the IR spectra. 1H NMR spectra were recorded at 298 K on a Varian 500 MHz instrument. All electronic absorption spectra were obtained with a scanning Cary 50 spectrophotometer (Varian). Magnetic moment measurements were made at 298 K with a magnetic susceptibility balance from Johnson Matthey (model MSB1), and (21) Sheldrick, G. M. Acta Crystallograph. 1990, A46, 467. (22) Rose, M. J.; Betterley, N. M.; Mascharak, P. K. J. Am. Chem. Soc. 2009, 131, 8340–8341.
Table 2. Selected Bond Distances (A˚) and Angles (deg) for 1, 2, 4, 7, and 8 bond distances
1
Fe-N1 Fe-N2 Fe-N(O) N-O Fe-Npy/DMAP Fe-S1 Fe-S2 Fe-Cl
1.9851(14) 1.9484(14) 2.2175(5) 2.2293(5) 2.3453(5)
Fe-N-O
-
2 1.989(3) 1.958(3) 1.682(3) 1.084(4) 2.2746(10) 2.2841(10) 177.1(3)
4
7
1.958(3) 1.968(3) 2.143(4) 2.1997(13) 2.2154(13) -
1.949(3) 1.965(3) 2.108(4) 2.1994(11) 2.2065(11) -
-
-
8 1.944(9) 2.007(11) 1.612(10) 1.167(11) 2.065(8) 2.273(5) 2.279(3) 173.2(8)
X-band EPR spectra were recorded at 125 K using a Bruker 500 ELEXSYS spectrometer. Magnetic susceptibility data for 7 and 8 were collected using a Quntum Design MPMS-XL SQUID magnetometer at temperatures ranging from 2 to 300 K under an applied magnetic field of 1000 G. Data were corrected for diamagnetic contributions using Pascal’s constants. Photolysis Experiments. The quantum yield value (φ) of NO photorelease from 9 was obtained using a tunable Apex Illuminator (150 W xenon lamp) equipped with a Cornerstone 130 1/8 M monochromator (measured intensity of ∼10 mW/cm2). The samples were prepared in a 1010 mm anaerobic quartz cuvette setup and placed 1 cm from the light source. Ferrioxalate was used as the actinometer in between 400 and 500 nm.23 Solutions were prepared at ∼0.5 mM to ensure sufficient absorbance (>90%) at the irradiation wavelength, and changes in electronic spectrum in the 450 nm region (
